US3301897A - Process for urea synthesis - Google Patents

Process for urea synthesis Download PDF

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Publication number
US3301897A
US3301897A US506752A US50675265A US3301897A US 3301897 A US3301897 A US 3301897A US 506752 A US506752 A US 506752A US 50675265 A US50675265 A US 50675265A US 3301897 A US3301897 A US 3301897A
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stream
gas
pressure
gas stream
urea
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US506752A
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Lucien H Cook
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Chemical Construction Corp
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Chemical Construction Corp
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Priority to US506752A priority Critical patent/US3301897A/en
Priority to GB48132/66A priority patent/GB1121900A/en
Priority to FR82776A priority patent/FR1498842A/fr
Priority to NL6615686A priority patent/NL6615686A/xx
Priority to BE689419D priority patent/BE689419A/xx
Priority to DE19661568348 priority patent/DE1568348A1/de
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    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C273/00—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups
    • C07C273/02—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups of urea, its salts, complexes or addition compounds
    • C07C273/04—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups of urea, its salts, complexes or addition compounds from carbon dioxide and ammonia

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  • the present invention relates to the synthesis of urea, by the reaction between ammonia and carbon dioxide at elevated temperature and pressure.
  • An improved complete off-gas recycle processing sequence is provided, in which the mixed oif-gas generated from the main process stream at a lower pressure level is adiabatically compressed to the next higher pressure level and is thereby heated to a highly elevated temperature.
  • the resulting hot oil-gas is mixed with the liquid process stream at the next higher pressure level, and thereby heats the liquid process stream with concomitant decomposition of ammonium carbamate.
  • the presence of the hot off-gas in contact with the liquid process stream also aids in the separation of further off-gas from the liquid phase during a subsequent indirect heating stage in which the liquid process stream is heated by heat exchange with a hot fluid such as steam.
  • the resultant cooling of the recycling otT-gas stream provides improved etficiency in the subsequent further compression of the off-gas prior to recycle to the urea synthesis reactor.
  • urea The synthesis of urea is commercially attained by the reaction between ammonia and carbon dioxide at elevated temperature and pressure. In most installations a complete recycle or total conversion process is desired, and consequently unreacted process components are recycled to the synthesis autoclave or reactor for further conversion. In this case, the process stream to the reactor will contain feed streams of ammonia and carbon dioxide, together with recycled process components.
  • the initial reaction between ammonia and carbon dioxide results in the formation of ammonium carbamate, The formation of ammonium carbamate takes place very rapidly, and the reaction goes essentially to completion.
  • the subsequent dehydration of ammonium carbamate to yield urea is an equilibrium reaction, and total conversion to urea is not attained in practice.
  • the efiluent stream from the synthesis reactor contains urea, ammonium carbamate, water, and excess free ammonia. Excess ammonia is usually provided to produce higher equilibrium conversion to urea, by a dehydration effect.
  • the reactor efiluent stream is usually heated in a plurality of stages at successively reduced pressure, in order to decompose ammonium carbamate and generate a mixed elf-gas containing ammonia, carbon dioxide and water vapor at each stage.
  • the unconverted process components in the off-gas must be recycled to urea synthesis, in order to provide a complete recycle process with total conversion of reactants.
  • Numerous procedures have been developed or proposed for the recycle of unconverted process components in urea synthesis. Thus, it has been proposed to recycle these components to the urea synthesis reactor as an aqueous ammonia-ammonium carbamate solution or slurry.
  • Another process is based on the use of hydrocarbon oil as the slurrying agent for ammonium carbamate.
  • an improved processing sequence involving the substantially adiabatic compression of the mixed off-gas stream prior to recycle is provided.
  • the urea synthesis reactor efiluent is usually heated in a plurality of stages at successively reduced pressure, in order to decompose ammonium carbamate and generate a mixed elf-gas at each stage.
  • the mixed off-gas generated from the main process stream at a lower pres sure level is compressed to the next higher pressure level in substantially adiabatic compression means, and is thereby heated to a highly elevated temperature.
  • the resulting hot off-gas stream is mixed with the liquid process stream at the next higher pressure level, and thereby heats the liquid process stream with concomitant decomposition of ammonium carbamate and evolution of further off-gas at this higher pressure level.
  • the resultant off-gas is then separated from the liquid phase and is further compressed, prior to eventual recycle to urea syn-thesis.
  • the procedure of the present invention provides several important advantages. Due to the direct contact mixing of the hot compressed off-gas with the liquid process stream at the higher pressure level, a portion of the heat content of the off-gas is usefully employed in further ammonium carbamate decomposition. In addition, this portion of the heat content of the olT-gas is utilized in a highly efiicient manner due to direct contact mixing, as compared to indirect heat exchange or heat transfer by the provision of an intermediary heat transfer agent.
  • the presence of the hot ofi-gas in contact with the liquid process stream also aid in the evolution and separation of further off-gas from the liquid phase during a subsequent indirect heating stage, in which the liquid process stream is heated by heat exchange with a hot fluid such as steam, by providing turbulence in the liquid phase.
  • the resultant cooling of the recycling off-gas stream is advantageous in providing improved efiiciency in the subsequent further compression of the off-gas prior to recycle to the urea synthesis reactor, because the gas stream is cooler.
  • Another object is to provide an improved complete recycle urea synthesis process, in which the olf-gas streams derived from ammonium carbamate decomposition are compressed and directly recycled to urea synthesis.
  • a further object is to decompose ammonium carbamate in the urea synthesis effluent stream and generate an olfgas in an improved manner.
  • An additional object is to efficiently utilize the hot olfgas produced by compression of olT-gas from a lower pressure level in substantially adiabatic compression means for decomposition of ammonium carbamate in the.
  • Still another object is to usefully and efficiently cool the hot recycling ofl-gas stream between stages of substantially adiabatic compression in a complete recycle urea synthesis process.
  • ammonia feed stream 1 which will usually consist of liquid ammonia, is passed into pump 2 and is compressed to urea synthesis pressure, typically in the range of 140 to 420 kg./sq. cm.
  • the resulting compressed ammonia feed stream 3 is added to compressed liquid or partially gaseous process stream 4 containing ammonia, carbon dioxide and water vapor.
  • stream 4 consists of the total process off-gas derived from ammonium carbamate decomposition, preferably combined with a gaseous carbon dioxide feed stream.
  • the mixed process stream 5 derived from the combination of streams 3 and 4 is now passed into urea synthesis reactor or autoclave 6, and is reacted at an elevated temperature typically in the range of 150 C. to 250 C. and elevated pressure typically in the aforementioned range of 140 to 420 kg./sq. cm., in order to form am monium carbamate by the initial reaction between am- 'monia and carbon dioxide, and to dehydrate a portion of the ammonium carbamate to yield urea.
  • the process stream in reactor 6 is usually cooled during the reaction, preferably by heat exchange with liquid water which is vaporized to produce usable process steam.
  • the liquid water stream 7 is passed into coil 8 within reactor 6, and is vaporized to produce steam stream 9.
  • the synthesis efiluent stream 10 derived from reactor 6 contains urea, ammonium carbamate, excess free ammonia and water, and will usually also contain a small proportion of inerts such as nitrogen.
  • the effluent stream 19 is preferably passed into retention vessel 11 which is maintained with an internal pressure substantially equal to the autogenous pressure of stream 10.
  • Vessel 11 is pro- Vided with an upper heat exchange section 12, through which a coolant is passed via streams 13 and 14.
  • the coolant 13 will preferably consist of liquid ammonia Which is at least partially vaporized in section 12, in which case stream 14 will consist of ammonia vapor.
  • the rising gas phase within vessel 11 passes upwards through heat exchange section 12, and the ammonia, carbon dioxide and water vapor components of the gas phase are condensed to liquid which passes downwards in vessel 11 and combines with the main liquid process stream 15 in the lower part of vessel 11.
  • the inerts gas com ponent of stream 10 is removed from the upper part of vessel 11 via stream 16 and is substantially free of process components.
  • Inerts stream 16 passes through pressure control valve 17 and is discharged to atmosphere via stream 18.
  • the resulting liquid process stream 19, now substantially free of inerts, is withdrawn from vessel 11 and passed through pressure reducing valve 20.
  • the resulting process stream 21, now at a reduced pressure typically in the range of 50 to 110 kg./sq. cm. and a temperature usually in the range of 50 C. to 250 C., is mixed with hot recycle off-gas stream 22, which is derived from a lower pressure stage as described infra and which will typically be at a temperature in the range of 300 C. to 550 C.
  • stream 22 serves to heat stream 21 and promotes decomposition of ammonium carbamate in the combined process stream 23, as described supra.
  • Stream 23 passes into heat exchanger 24 provided with internal tubes 25, which are externally heated by hot fluid stream 26.
  • Stream 26 will preferably consist of steam, in which case the resultant cooled fluid stream 27 will consist of condensate water.
  • the heating of stream 23 in tubes produces further decomposition of ammonium carbamate and generation of olf-gas consisting of ammonia and carbon dioxide, together with vaporized water.
  • the presence of recycle gaseous stream 22 within the tubes 25 serves to promote the evolution of ofl-gas and separation of off gas from the liquid phase.
  • the resultant mixed gasliquid stream 28 discharged from unit 24 is passed into gas-liquid separator 29, which is a baffled or cyclonic means of conventional design for separation of the gas phase from the liquid phase.
  • the resulting combined olT-gas stream removed from separator 29 contains ammonia, carbon dioxide and Water vapor components derived from stream 22, as well as similar components derived from the decomposition of ammonium carbamate and vaporization of water from stream 21.
  • stream 30 is now recycled to urea synthesis.
  • stream 30 is passed into gas compressor 31, which will preferably consist of substantially adiabatic compression means as described in U.S. Patent No. 3,200,148, and the gas stream is compressed in unit 30 to elevated urea synthesis pressure and a highly elevated temperature, typically in the range of 300 C. to 550 C.
  • the resultant hot gas stream 32 discharged from unit 31 is passed through coil 33 in heat exchanger 34, and is cooled to a suitable temperature for subsequent urea synthesis. Partial or total condensation of the gas stream to the liquid phase may occur due to the cooling in coil 33.
  • the recycle process stream 4 discharged from coil 33 is processed as described supra, for further urea synthesis.
  • a heat exchange fluid stream is passed into unit 34, and serves to cool the coil 33.
  • the resultant heated fluid is discharged from unit 34 as stream 36. In most instances, stream 35 will consist of liquid water and stream 36 will consist of usable process steam.
  • stream 37 the residual liquid phase derived from stream 28 is removed as stream 37, which is passed through pressure reducing valve 38.
  • the resulting process stream 39 now at a reduced pressure typically in the range of 10 to 40 kg./sq. cm. and a temperature usually in the range of 50 C. to 250 C., is mixed with hot recycle off-gas stream 40, which is derived from the lowest pressure stage as described infra and which will typically be at a temperature in the range of 300 C. to 550 C.
  • stream 40 serves to heat stream 39 and pro motes decomposition of ammonium carbamate in the combined process stream 41, as described supra.
  • Stream 41 passes into heat exchanger 42, which is similar to unit 24 described supra.
  • unit 42 is provided with internal tubes 43, which are externally heated by hot fluid stream 44.
  • Stream 44 will preferably consist of steam, in which case the resultant cooled fluid stream 45 will consist of condensate water.
  • the heating of stream 41 in tubes 43 produces further decomposition of ammonium carbamate and generation of off-gas consisting of ammonia and carbon dioxide, together with vaporized water.
  • the presence of recycle gaseous stream 40 within the tubes 43 serves to promote the evolution of offgas and separation of olT-gas from the liquid phase.
  • the resultant mixed gas-liquid stream 46 discharged from unit 42 is passed into gas-liquid separator 47, which is a unit similar in configuration and function to unit 29 described supra.
  • the resulting combined off-gas stream 48 removed from separator 47 contains ammonia, carbon dioxide and water vapor components derived from stream 40, as well as similar components derived from the decomposition of ammonium carbamate and vaporization of water from stream 39.
  • Stream 48 is now compressed to an elevated pressure typically in the range of 50 to kg./sq. cm. in substantially adiabatic gas compressor 49, which consists of compression means as described in US. Patent No. 3,200,148.
  • the gas stream is thus heated to a highly elevated temperature, typically in the range of 300 C. to 550 C., and is discharged from unit 49 and recycled as stream 22.
  • the residual liquid phase derived from stream 46 is removed as stream 50, which is passed through pressure reducing valve 51.
  • the resulting process stream 52 now at a reduced pressure typically in the range of 0.35 to 3.5 kg./sq. cm. and a temperature usually in the range of 50 C. to 250 C., is passed into heat exchanger 53, which is similar to unit 24 described supra.
  • unit 53 is provided with internal tubes 54, which are externally heated by hot fluid stream 55.
  • Stream 55 will preferably consist of steam, in which case the resultant cooled fluid stream 56 will consist of condensate water.
  • the heating of stream 52 in tubes 54 produces final decomposition of ammonium carbamate and generation of off-gas consisting of ammonia and carbon dioxide, together with vaporized water.
  • the resultant mixed gas-liquid stream 57 discharged from unit 53 is passed into gas-liquid separator 58, which is a unit similar in configuration'and function to unit 29 described supra.
  • the residual liquid phase derived from stream 57 is removed from unit 58 as stream 59, which consists of product aqueous urea solution substantially free of ammonia and carbon dioxide.
  • Stream 59 is now passed to product utilization, such as evaporative concentration to produce solid crystal urea or concentration followed by prilling to produce solid urea prills.
  • a low pressure off-gas stream 60 is removed and contains ammonia, carbon dioxide and water vapor components derived from stream 57.
  • Stream 60 is now preferably combined with carbon dioxide feed stream 61, in accordance with US patent application No. 491,675 mentioned supra.
  • the resulting combined gaseous stream 62 is compressed to an elevated pressure typically in the range of to 40 kg./ sq. cm. in substantially adiabatic gas compressor 63, which consists of compression means as described in US. Patent No. 3,200,- 148.
  • the gas stream is thus heated to a highly elevated temperature, typically in the range of 300 C. to 550 C., and is discharged from unit 63 and recycled as stream 40.
  • the process concept of the present invention is applicable to any urea synthesis process in which ammonium carbamate decomposition is carried out in a plurality of stages at successively reduced pressure, although optimum efficiency is attained in most instances when the ammonium carbamate decomposition and olT-gas generation is carried out in three or more stages at successively reduced pressure, since in this manner compression power requirements are reduced due to conservation of pressure levels of oiT-gas.
  • the inert gaspurger 11 may be of an alternative configuration similar to unit 71 described in US. patent application No. 246,747, now US. Patent No. 3,232,985, mentioned supra.
  • Other alternative arrangements or means for removal of inert gas from the system such as periodically purging a portion of the recycling off-gas or the proc ess disclosed in US. Patent No. 2,214,068, will occur to those skilled in the art.
  • the process concept of US. Patent No. 3,172,911 may be utilized in conjunction with the present invention.
  • the process streams 21 or 39 will be initially passed to an auxiliary gas-liquid separator similar to unit 29, to separately remove the gaseous phase generated by adiabatic flash expansion to the lower pressure level. This gaseous phase will then be added to the respective olT-gas stream 30 or 48 for recycle to urea synthesis, and the residual liquid phase will be mixed with hot recycling off-gas stream 22 or 40 in accordance with the present invention.
  • greater efiiciency in off-gas compression and recycle may be attained by initially reducing the pressure of stream 19 to an intermediate pressure above that of stream 21, and separating an initial oif-gas generated by adiabatic flash expansion. The pressure level of the residual liquid phase would then be further reduced to produce stream 21.
  • stream 32 would be produced at a pressure level equal to that of the initial offgas, and the two gas streams would be combined and further compressed to urea synthesis pressure and passed to coil 33.
  • stream 32 could be produced at urea synthesis pressure and the initial off-gas could also be separately compressed to urea synthesis pressure and combined with stream 32, and the combined gas stream would then be passed to coil 33.
  • stream 22 In order to provide accurate temperature control, in some cases it will be desirable to divide stream 22 into two portions, one of which is mixed with stream 21 as described supra, with the other portion being directly added to stream 30 for further compression. Similar considerations apply with respect to stream 40, thus in practice stream 40 may be divided into two portions, one of which is mixed with stream 39 as described supra, with the other portion being directly added to stream 48 for further compression.
  • unit 34 may be omitted and stream 32 may alternatively be directly recycled as stream 4 without prior cooling, since the addition of liquid ammonia stream 3 to the gaseous process stream provides a cooling eifect.
  • streams 9 and 36 consisting of usable process steam may be totally or partially employed as heating steam for ammonium carbamate decomposition, in which case streams 26, 44 and 55 would be derived from streams 9 or 36.
  • the gaseous carbon dioxide feed stream 61 will usually be derived froma process source in which carbon dioxide is produced at relatively low pressure, such as by-product carbon dioxide from the production of ammonia synthesis gas.
  • the initially available carbon dioxide will be compressed in additional compressor means, prior to the addition to stream 60 as stream 61.
  • the gaseous carbon dioxide feed stream 61 may be available at an elevated temperature level.
  • a portion or all of stream 61 may be added to liquid stream 52, to obtain advantageous results similar to the improved results of the present invention'derived from the addition of hot gaseous streams 22 or 40 to the liquid streams 21 or 39 respectively.
  • the carbon dioxide feed stream 61 may be initially available at elevated pressure. In such circumstances, stream 61 may be added to streams 48 or 30. In other instances, stream 61 may be available at elevated pressure and high temperature, in which case stream 61 may be added to streams 40 or 22. Stream 61 may even be separately compressed to urea synthesis pressure and added directly to recycle stream 4, as described in US. Patent 3,200,148. In all such instances, where stream 61 is available at elevated pressure or is separately compressed prior to addition to the process, the low pressure recycle gas stream 62 will consist solely of offgas stream 60 derived from decomposition of ammonium carbamate in stream 52.
  • Example 1 The process concepts of the present invention were applied with respect to the design of a 1000 tons/ day urea plant.
  • the improvement in efliciency was shown with respect to the prior art sequence consisting of the direct mixing of stream 40 with the off-gas derived only from ammonium carbamate decomposition at the next higher pressure level, followed by further gas compression.
  • This prior art sequence which would essentially consist of adding stream 40 to stream 48, is designated as Case A.
  • the process sequence of the present invention, in which stream 40 is mixed with stream 39, is designated as Case B. Following is a tabulation of pertinent operating conditions and energy requirements for both cases.
  • Case B resulted in a power saving of 9.55% in the operation of compressor 49, as compared to Case A.
  • Case B attained a steam saving of 14% in the operation of decomposer 42, as compared to Case A.
  • step (b) mixing the hot compressed off-gas stream from step (a) with the liquid process stream at the next higher pressure stage, whereby said off-gas stream heats said liquid process stream and thereby promotes decomposition of ammonium carbamate,
  • step (c) separating a combined off-gas stream, derived from components of the off-gas stream from step (a) together with components derived from decomposition of ammonium carbamate and vaporization of water from said liquid process stream, from the residual liquid process stream, and
  • step (d) further compressing the combined ofl gas stream derived from step (c) to a higher pressure, prior to recycle of the total process off-gas as said recycled process stream.
  • step (a) is heated to a temperature in the range of 300 C. to 550 C. by said oil-gas compression, and the liquid process stream of step (b) is at an initial temperature in the range of 50 C. to 250? C. prior to mixing with said hot compressed off-gas stream.
  • step (b) mixing the hot compressed otf-gas stream from step (a) with the liquid process stream at the next higher pressure stage and prior to said heating of the liquid process stream by indirect heat exchange with a'hot fluid, whereby said off-gas stream heats said liquid process stream and thereby promotes decomposition of ammonium carbamate,
  • step (c) separating a combined olT-gas stream, derived from components of the off gas stream from step (a) together with components derived from decomposition of ammonium carbamate and vaporization of water from said liquid process stream, from the residual liquid process stream after said heating by indirect heat exchange with a hot fluid, and
  • step ((1) further compressing the combined oflagas stream derived from step (c) to a higher pressure, prior to recycle of the total process off-gas as said recycled process stream.
  • step (a) is heated to a temperature in the range of 300 C. to 550 C. by said off-gas compression, and the liquid process stream of step (b) is at an initial temperature in the range of 50 C. to 250 C. prior to mixing with said hot compressed off-gas stream.

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  • Organic Chemistry (AREA)
  • Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
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US506752A 1965-11-08 1965-11-08 Process for urea synthesis Expired - Lifetime US3301897A (en)

Priority Applications (6)

Application Number Priority Date Filing Date Title
US506752A US3301897A (en) 1965-11-08 1965-11-08 Process for urea synthesis
GB48132/66A GB1121900A (en) 1965-11-08 1966-10-18 Urea synthesis process
FR82776A FR1498842A (fr) 1965-11-08 1966-11-07 Procédé de synthèse de l'urée
NL6615686A NL6615686A (enrdf_load_stackoverflow) 1965-11-08 1966-11-07
BE689419D BE689419A (enrdf_load_stackoverflow) 1965-11-08 1966-11-08
DE19661568348 DE1568348A1 (de) 1965-11-08 1966-11-08 Verfahren zur Herstellung von Harnstoff

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US506752A US3301897A (en) 1965-11-08 1965-11-08 Process for urea synthesis

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US3301897A true US3301897A (en) 1967-01-31

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US (1) US3301897A (enrdf_load_stackoverflow)
BE (1) BE689419A (enrdf_load_stackoverflow)
DE (1) DE1568348A1 (enrdf_load_stackoverflow)
FR (1) FR1498842A (enrdf_load_stackoverflow)
GB (1) GB1121900A (enrdf_load_stackoverflow)
NL (1) NL6615686A (enrdf_load_stackoverflow)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3370090A (en) * 1965-09-30 1968-02-20 Chemical Construction Corp Urea synthesis process
US4092358A (en) * 1968-09-03 1978-05-30 Snamprogetti S.P.A. Process for the production of urea having a low carbamate content
WO2003010089A1 (en) * 2001-07-24 2003-02-06 Dsm Ip Assets B.V. Method for obtaining an ammonium carbamate solution from a gas mixture containing nh3, h2o and co2

Families Citing this family (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS4919259B1 (enrdf_load_stackoverflow) * 1969-08-30 1974-05-16
JPS56128749A (en) * 1980-03-13 1981-10-08 Mitsui Toatsu Chem Inc Stripping of unreacted material in urea preparation process
JPS5750954A (en) * 1980-09-12 1982-03-25 Mitsui Toatsu Chem Inc Synthesis of urea

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
None *

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3370090A (en) * 1965-09-30 1968-02-20 Chemical Construction Corp Urea synthesis process
US4092358A (en) * 1968-09-03 1978-05-30 Snamprogetti S.P.A. Process for the production of urea having a low carbamate content
WO2003010089A1 (en) * 2001-07-24 2003-02-06 Dsm Ip Assets B.V. Method for obtaining an ammonium carbamate solution from a gas mixture containing nh3, h2o and co2
US20040199013A1 (en) * 2001-07-24 2004-10-07 Lardinois Guillame Mario Hubert Jozef Method for obtaining an ammonium carbamate solution from a gas mixture containing nh3, h2o and co2
US6914157B2 (en) 2001-07-24 2005-07-05 Dsm Ip Assets B.V. Method for obtaining an ammonium carbamate solution from a gas mixture containing NH3, H2O and CO2

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BE689419A (enrdf_load_stackoverflow) 1967-05-08
FR1498842A (fr) 1967-10-20
DE1568348A1 (de) 1970-02-12
NL6615686A (enrdf_load_stackoverflow) 1967-05-09
GB1121900A (en) 1968-07-31

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